Meiosis & Sexual Reproduction (Ch. 11)
Cell division / Asexual reproduction Mitosis produce cells with same information identical daughter cells exact copies clones same DNA Aaaargh! I m seeing double!
Asexual reproduction Single-celled eukaryotes yeast (fungi) Protists Paramecium Amoeba Simple multicellular eukaryotes Hydra budding budding What are the disadvantages of asexual reproduction? What are the advantages?
How about the rest of us? What if a complex multicellular organism (like us) wants to reproduce? joining of egg + sperm Do we make egg & sperm by mitosis? What if we did, then. No! 46 + 46 92 egg sperm zygote Doesn t work!
46 chromosomes 23 pairs Human female karyotype
46 chromosomes 23 pairs Human male karyotype
Homologous chromosomes Paired chromosomes both chromosomes of a pair carry matching genes control same inherited characters homologous = same information diploid 2n 2n = 4 single stranded homologous chromosomes double stranded homologous chromosomes
How do we make sperm & eggs? Must reduce 46 chromosomes 23 must reduce the number of chromosomes by half 46 meiosis 23 egg 23 23 zygote 46 46 23 fertilization sperm gametes
Meiosis: production of gametes Meiosis chromosome number must be reduced diploid haploid 2n n humans: 46 23 makes gametes fertilization restores chromosome number haploid diploid n 2n haploid diploid
Sexual reproduction lifecycle 2 copies diploid 2n 1 copy haploid 1n fertilization In the next generation We re mixing things up here! A good thing? meiosis 1 copy haploid 1n gametes gametes
Meiosis Reduction Division special cell division for sexual reproduction reduce 2n 1n diploid haploid two half makes gametes sperm, eggs Warning: meiosis evolved from mitosis, so stages & machinery are similar but the processes are radically different. Do not confuse the two!
Overview of meiosis I.P.M.A.T.P.M.A.T 2n = 4 interphase 1 prophase 1 metaphase 1 anaphase 1 n = 2 n = 2 prophase 2 metaphase 2 anaphase 2 telophase 2 n = 2 telophase 1
Double division of meiosis DNA replication 1st division of meiosis separates homologous pairs 2nd division of meiosis separates sister chromatids Meiosis 1 Meiosis 2
Meiosis 1 1st division of meiosis separates homologous pairs prophase 1 2n = 4 single stranded 2n = 4 double stranded synapsis metaphase 1 2n = 4 double stranded tetrad telophase 1 1n = 2 double stranded reduction
Meiosis 2 2nd division of meiosis separates sister chromatids 1n = 2 double stranded What does this division look like? 1n = 2 double stranded prophase 2 metaphase 2 1n = 2 single stranded 4 telophase 2
Meiosis 1 & 2
Trading pieces of DNA Crossing over during Prophase 1, sister chromatids intertwine homologous pairs swap pieces of chromosome DNA breaks & re-attaches prophase 1 synapsis tetrad
Crossing over 3 steps cross over breakage of DNA re-fusing of DNA New combinations of traits What are the advantages of crossing over in sexual reproduction?
Mitosis vs. Meiosis
Mitosis vs. Meiosis Mitosis 1 division daughter cells genetically identical to parent cell produces 2 cells 2n 2n produces cells for growth & repair no crossing over Meiosis 2 divisions daughter cells genetically different from parent produces 4 cells 2n 1n produces gametes crossing over
Putting it all together meiosis fertilization mitosis + development gametes 46 23 meiosis 46 egg 46 23 sperm 23 23 46 46 46 46 46 46 46 46 zygote fertilization mitosis development
The value of sexual reproduction Sexual reproduction introduces genetic variation genetic recombination independent assortment of chromosomes random alignment of homologous chromosomes in Metaphase 1 crossing over mixing of alleles across homologous chromosomes random fertilization which sperm fertilizes which egg? providing variation for natural selection metaphase1
Variation from genetic recombination Independent assortment of chromosomes meiosis introduces genetic variation gametes of offspring do not have same combination of genes as gametes from parents random assortment in humans produces 2 23 (8,388,608) different combinations in gametes from Mom from Dad offspring new gametes made by offspring
Variation from crossing over Crossing over creates completely new combinations of traits on each chromosome creates an infinite variety in gametes
Variation from random fertilization Sperm + Egg =? any 2 parents will produce a zygote with over 70 trillion (2 23 x 2 23 ) possible diploid combinations (not even counting crossing over!!!!!!!!!)
Sexual reproduction creates variability Sexual reproduction allows us to maintain both genetic similarity & differences. Jonas Brothers Baldwin brothers Martin & Charlie Sheen, Emilio Estevez
Sperm production Epididymis Testis Coiled seminiferous tubules germ cell (diploid) Vas deferens primary spermatocyte (diploid) secondary spermatocytes (haploid) spermatids (haploid) MEIOSIS I MEIOSIS II Spermatogenesis continuous & prolific process Cross-section of seminiferous tubule each ejaculation = 100-600 million sperm spermatozoa
Egg production Oogenesis eggs in ovaries halted at Metaphase 1 Meiosis 1 completed during maturation Meiosis 2 completed after fertilization 1 egg + 2 polar bodies unequal divisions Meiosis 1 completed during egg maturation What is the advantage of this development system? Meiosis 2 completed triggered by fertilization ovulation
Oogenesis Putting all your egg in one basket! germinal cell (diploid) primary follicles fallopian tube fertilization MEIOSIS I first polar body MEIOSIS II after fertilization second polar body primary oocyte (diploid) secondary oocyte (haploid) ovum (haploid) developing follicle mature follicle with secondary oocyte ruptured follicle (ovulation) corpus luteum
Differences across kingdoms Not all organisms use haploid & diploid stages in same way which one is dominant (2n or n) differs but still alternate between haploid & diploid must for sexual reproduction
Eggs are Precious, Sperm is Cheap. Any Questions?? What are the DISadvantages of sexual reproduction?
Review Questions
1. How do cells at the completion of meiosis compare with cells that have replicated their DNA and are just about to begin meiosis? A. They have twice the amount of cytoplasm and half the amount of DNA. B. They have half the number of chromosomes and half the amount of DNA. C. They have the same number of chromosomes and half the amount of DNA. D. They have half the number of chromosomes and onefourth the amount of DNA. E. They have half the amount of cytoplasm and twice the amount of DNA.
2. Which number represents G 2? * A. I B. II C. III D. IV E. V
3.. Which number represents the DNA content of a sperm cell? A. I B. II C. III D. IV E. V
4. Which of the following would not be considered a haploid cell? A. daughter cell after meiosis II B. gamete C. daughter cell after mitosis in gametophyte generation of a plant D. cell in prophase I E. cell in prophase II
5. A cell in G 2 before meiosis compared with one of the four cells produced by that meiotic division has A. twice as much DNA and twice as many chromosomes. B. four times as much DNA and twice as many chromosomes. C. four times as much DNA and four times as many chromosomes. D. half as much DNA but the same number of chromosomes. E. half as much DNA and half as many chromosomes.